Single-stage AC-DC power conversion is a low cost and thus popular power supply topology for applications such as solid-state lighting. An important parameter for a single-stage AC-DC power switching converter is its power factor, which is the ratio of the real power delivered by the AC mains to the single-stage AC-DC switching power converter as compared to the apparent power delivered to the single-stage AC-DC switching power converter. The apparent power is insensitive to the phasing between the input current and voltage in contrast to the real power. The power factor (PF) is thus lowered if the input current and voltage are out of phase. The rectified input voltage to a single-stage AC-DC switching power converter cycles from approximately zero volts to the peak line voltage (e.g., 120 V*1.414 in the US) at twice the frequency for the AC mains. Given this sinusoidal pulsing or cycling of the rectified input voltage, the input current should have a similar profile to achieve a high PF such as by the use of a suitably-modified peak current or constant on time control methodology.
In either of these techniques, the switching power converter regulates the cycling of the power switch transistor so that the input current to the switching power converter during periods of high load has a profile that is in-phase with the profile or envelope for the rectified input voltage. Each cycle of the rectified input voltage begins with a relatively-low voltage (e.g. zero volts) to reach a peak voltage mid-cycle and then falls again to the relatively-low voltage. To achieve a high PF, the peak value for each cycle of the input current to the switching power converter will have a profile or threshold envelope that is similar to the rectified input voltage's envelope. The peak input current will thus cycle in phase with the rectified input voltage so that the peak input current will be relatively small at the beginning of a cycle, pass through a peak mid-cycle, and then fall again to a relatively-small value at the end of each cycle.
In achieve high efficiency, it is also conventional for the controller to alter the modulation mode of the cycling of the power switch transistor depending upon the load. During periods of high load, a pulse width modulation mode may be used. But as the load drops, it is conventional to transition to a pulse frequency modulation mode. While pulse frequency modulation is used, the switching frequency drops as the load decreases and increases as the load increases. The resulting drop in the pulse switching frequency during low-load pulse frequency modulation operation can cause a number of problems. For example, the switching frequency for the cycling of the power switch transistor may enter the audible range. In addition, the switching frequency is independent of the AC mains cycle that drives the sinusoidal profile for the rectified input voltage. At the beginning and end of each cycle for the rectified input voltage, it is desirable for the peak current for each cycle of the power switch transistor to be relatively small so that the profile of the input current to the switching power converter is in in-phase with the envelope of the rectified input voltage. But the peak current must also be responsive to the output voltage. To maintain the output voltage within regulation, the peak current must be undesirably high during the beginning and end of each cycle of the rectified input voltage (the zero-crossing times for the AC mains input voltage). Conventional pulse frequency modulation during low load conditions thus suffers from a reduced power factor due to the need to increase the peak current for the power switch in the vicinity of the zero-crossing times for the AC mains input voltage.
Accordingly, there is a need in the art for single-stage power converters having robust power factor correction during pulse frequency modulation operation.